The basic for rapid transcriptional activation in response to extracellular stimuli is a fundamental question for cell biology. Certainly such activation in response to lymphokine stimulation is critical for an appropriate immune response. The interferon system affords an attractive model with which to address mechanistic aspects of this question, especially as it displays multiple levels of specificity. The goal of this proposal is to define the mechanisms of gene activation by one class of interferon, IFN-gamma. A number of the early-response genes involved in macrophage activation are induced by IFN-gamma but not IFN-alpha. Among these are the genes encoding the high affinity receptor for lgG, FcgammaRI. We have found that a 39 nucleotide DNA sequence, the IFN-gamma response region (GRR), is present in all FcgammaRI promoters characterized thus far, and is necessary and sufficient for transcriptional induction of those promoters by IFN-gamma. We have also found that a 91 kDa protein, fist identified as a component of the IFN-alpha induced transcription complex, ISGF3, will assemble at the 3' end of the GRR in response to either IFN-gamma or IFN-gamma. As neither FcgammaRI nor GRR-reporter constructs are induced by IFN-alpha, the IFN-gamma specificity of these promoters appears to arise from additional proteins which interact with the GRR following IFN-gamma exposure. Evidence supporting such interactions includes UV crosslinking experiments which show specific binding of a 40- 50 kDa protein of the GRR, and mutational analysis of the GRR demonstrating a protein complex which assembles at the 5' end, interacts with 91, and is necessary for optimal responsiveness to IFN-gamma. The goal of our proposal is to identify and characterize the proteins that mediate transcriptional induction by IFN-gamma at the GRR.